1. Field of the Invention
The method of the present invention relates in general to the field of synthesizing one or more colors directly on color sensitive photographic media by the exposure thereof to an illuminant source through a group or set of color filter elements. More particularly, the method of the present invention relates to the reproduction of a desired color (the target color) by illuminating the target color with a light source and determining the exposure time needed for each color filter of a selected set of color filter elements to reproduce the target color by exposing a photographic medium to the same or like light source through said set of color filter elements. One application of the method is the preparation of single sheet pre-press color proofs from color separation films, masks or the like.
2. Description of the Prior Art
In a classic experiment which was performed by Newton as early as the year 1730, light from a test lamp was shown on a white screen and viewed by an observer. An adjacent part of the screen was illuminated by three different light sources equipped to give light of widely differing colors, typically, red, green and blue. Interestingly, it was found that by adjusting the intensities of the three colored light sources, they could be made to produce a combined light on the screen which would match that of the test lamp. This basic experiment teaches that light from three different colored light sources, commonly referred to as the primary lights, can combine to reproduce a test color by a technique known as additive color matching.
Following the advent of color photography, it was noted that shining primary lights of different colors on the same piece of photographic media produced an image thereon of combined color which was different from the color of each individual primary light. Such a concept is disclosed in U.S. Pat. No. 3,741,649, issued June 26, 1973, to Podesta et al. However, Podesta discloses merely an empirical system whereby, through trial and error, light is projected through a multitude of combinations of filters onto photographic media which is then developed to discover what usable color, if any, has been obtained. By noting which filters were used, some rudimentary repeatability of results was attainable. However, the method disclosed by Podesta makes no provision for reproducing a randomly selected target color and makes no provision for duplicating such a target color when a light source having a different color temperature is used, or when filters are used which are of colors different from those already used.
U.S. Pat. No. 3,322,025, issued May 30, 1967, to Dauser, discloses a somewhat more sophisticated, but still empirical method which involves the production of a cylindrical color solid wherein each colored segment is identified by its hue, value and intensity. The color solid is generated, apparently, by varying the voltages to three primary light sources, red, green and blue, thereby to regulate their intensities and thus produce the various colored segments comprising the color solid. Repeatability is obtained by noting the voltage input or light output of each of the respective lamps needed to produce each colored segment in the color solid.
However, such a system is severely limited since it provides only for producing the cylindrical color solid by the use of three colored lights which together are able to form achromatic light. If it is desired to use three lights which are unable to form achromatic light, what technique to use is undisclosed. Further, if a different set of achromatic lights is used, it is clear that the first produced color solid is useless and that a new color solid must be prepared for each different set of lights employed. In addition, the method's ability to faithfully reproduce a color segment is questionable since it is well known that the color properties of photographic media vary from manufacturer to manufacturer and from lot to lot. How Dauser would compensate for such variations without regenerating a complete color solid for each batch of photographic paper used is not disclosed. In addition, it is well known that the color temperature of even standardized light sources changes with age and hence the colors they produce similarly change. Again, how such aging is compensated for, without the periodic reconstruction of a complete color solid, is a problem not addressed by Dauser. Further, it is well known that varying the voltage to a light source is a crude way to regulate its intensity since as the bulb dims or brightens, the color produced by the bulb also changes. Apparently, Dauser failed to consider this variable which would tend to make his color solid nonlinear. Lastly, if both positive and negative photographic media were to be used, apparently a separate color solid would have to be generated for each, a costly and time-consuming process.
U.S. Pat. No. 2,850,563, issued Sept. 2, 1958, to Gretener uses a chromaticity diagram as a theoretical basis for explaining his method of more accurately reproducing images in color through the use of photography or television. Primarily, his method achieves accurate color reproduction by properly adjusting the spectral response of the recording apparatus. For reproducing flesh tones, the spectral response is adjusted in such a manner that the flesh tones are recorded with color components having an intensity of equal size, i.e., having a unitary ratio, or in the preferred case, with only the red and green components having substantially equal intensities. It is noted that the thrust of the Gretener patent is directed towards a method of unitizing the ratio of at least two of the primary colors.
The Gretener patent fails, however, to disclose a method by which a target color may be produced on positive photographic media by calculating the exposure time needed of suitable color filter elements representative of each of the primary color components of the target color. In addition, no method is disclosed in Gretener by which one may calculate the proper exposure times for each of the selected color filter elements using a given light source such that when negative photographic media is exposed thereby, the target color will result on the developed media.
The color reproduction technique of the present invention also is applicable to the color printing field, particularly to the preparation of color proofs from color separation components in the form of films, masks or the like. Conventional practice with respect to color proofing involves either actual press proofing (a procedure wherein actual test printing plates are made to literally color print the proof) or by use of primary color and black transparent overlays. 3M Company markets, under the trademark "Color-Key", pre-press proofing materials involving pre-sensitized ink pigment coatings, in either transparent or opaque colors, on transparent polyester base sheets. Each pigment coating is intended to be correlated with various process color printing inks. Each primary color and black "Color-Key" Sheet is overlaid by its associated separation negative and after exposure and development, the four "Color-Key" sheets are overlaid in register to provide a "proof" or simulation of what the four-color work will look like when printed. Manifestly, such a pre-press proofing system, involving four overlaid sheets (which are glossy in character and which at best only indirectly match the colors of the proof with the actual colors of the printing inks to be used) falls well short of providing the user with a fully accurate proof in the sense of the colors, color registry and texture of the color work when press printed with inks on paper.
3M Company also markets a pre-press color proofing system under the trademark "Transfer-Key", which is said to provide a complete four-color proof on a single sheet. In this system, factory pre-coated carrier sheets of color pigment, respectively bearing cyan, yellow, magenta and black pigment, are successively manipulated to laminiate each pigment onto the base material by use of a proprietary laminator. Specifically, the cyan pigment is first laminated to the base material, which is then exposed to the cyan separation negative and the sheet then developed in a proprietary processor. The same exposure/development cycle is repeated with each color, producing the four-color proof. Pre-coated pigment carrier sheets are available only in certain colors, unless specially ordered. As manifest, such color proofing procedure, although providing a single sheet color proof, is of limited applicability and accuracy in that only certain laminating pigments are available, and is inherently rather slow in cycle time in that each pigment layer of the four-color proof must be separately and successively laminated, exposed and developed.
Another commercially available pre-press color proofing system is marketed by the Dupont Company, under the trademark "Cromalin." In the Cromalin color proof system, dry pigment toners are factory "calibrated" to printing ink colors and each primary and black color reproduction is on a separate sheet of photopolymer film, the films after exposure and development being laminated together in registry to provide the color proof. The cycle time for this system is said to be "within an hour", rather than the hours or days required for press proofing.
Another known pre-press color proofing system which is commercially available is the Kodak Polytrans Colour Proofing Film System, such as described in "The Reproduction of Color", by R. W. G. Hunt, at page 546 (published by Fountain Press of Hertfordshire, England, Third Edition, 1975). In the Polytrans systems screened color separation films are proofed by materials including films consisting of color-pigmented photopolymer matrices coated on a transparent film base. Exposure to blue or ultraviolet light hardens the photopolymer so that exposure thereof to the screened separations results in an image-wise pattern of the hardened polymer being obtained. Each exposed film is mounted on a suitable roller and rolled under heat and pressure onto a sheet of paper, the unhardened polymer transferring to the paper and taking the pigment with it. Four different colors of film are used and, by exposing each to the corresponding color separation positive, image-wise transfers can be obtained in each color. Each image is developed and transferred in succession, onto the paper, in register, with a color proof resulting after exposure of the transferred pigments to light. While the Polytrans system results in a color proof on a single sheet of paper, the color-pigmented photopolymer matrices used are necessarily precalibrated to printer's inks at the factory, and each color is separately and successively developed on the color proof. Registry of the respective color images on the paper can also prove to be a problem because the color images are applied to the paper by successive roller transfer.